The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared negative working printing plate is moistened with water and an ink is then applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The reverse holds true for positive working plates, in which the background is imaged. The ink is then transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are less common, yet represent a steadily growing market. Currently, most of these plates utilize similar materials and similar imaging mechanisms as UV-imageable plates. For example, in a resole-based plate, a thermal acid generator might be used in lieu of a photoacid generator and the same series of preheat and development steps might be employed. The main advantage of these digital plates is that the thermal imaging process is rapid and inexpensive compared to the analog process involving the creation of a mask and blanket UV exposure.
It is a desired aim in the lithographic printing industry to make this process even more rapid, cost effective, and efficient by eliminating the processing steps altogether. In the ideal situation, a plate would be imaged, placed directly on press, and immediately used. Plates of this type which image by an ablation method are known to those skilled in the art. The imaging of ablation plates typically causes the generation of unwanted material which can foul the optics of the imaging device and which must often be removed from the surface of the plate. This introduces the problem of collection and disposal ofthe extraneous material.
Accordingly, it would be desirable to employ an imaging member in a processless thermal lithographic printing process wherein the member was capable of being imaged without the attendant problem of material removal. It is one object of this invention to provide such an imaging member. The imaging member of this invention contains at least one switchable polymer. By "switchable" as referred to herein, it is meant that the polymer is rendered from hydrophobic to relatively more hydrophilic, or conversely, from hydrophilic to relatively more hydrophobic, upon exposure to heat. Such polymers will undergo thermally driven chemical reactions in which highly polar moieties are either created or destroyed under imaging conditions. This results in the storage of the imaging data as hydrophilic and hydrophobic regions of a continuous polymer surface. In addition to being completely processless, such plates have the advantage of not needing any type of material collection devices which ablation-based plates require. Also unlike ablation plates, a switchable polymer plate in its ideal form would consist of one layer and can be manufactured on a single pass through a coater.
Though there are many such chemical reactions which may be of potential use for such application, it has been found that many of the reactions which would result in large polarity differentials between the background and foreground will be too slow under standard imaging conditions. Conversely, many reactions which are the most facile under thermal imaging conditions will also show some reactivity at ambient temperatures. Materials which react by these mechanisms, thus, will have a very short shelf life and will be unsuitable for commercial applications.
In view of the foregoing, it is another object of this invention to provide an imaging member containing at least one switchable polymer where the "switching" of the polymer is driven by chemical reactions which occur rapidly under thermal imaging conditions yet the polymer itself has low enough reactivity at ambient temperatures to have a useful shelf life.